The present invention basically relates to a multi-layer insulation system for electrical conductors, an insulated electrical conductor, a process for preparing an insulated conductor, and an insulated conductor prepared by such a process. The insulated electrical conductors of the present invention are lightweight, qualify for temperature ratings of up to approximately 230xc2x0 C., and demonstrate mechanical durability, and hydrolysis resistance. As such, these insulated conductors are particularly useful for aircraft wire and cable.
Electrical insulation must meet a variety of construction and performance requirements. These requirements are particularly severe for electrical cable which is to be used in aircraft and similar equipment. Electrical cable useful for such applications must demonstrate a balance of electrical, thermal, and mechanical properties, with overall performance being evaluated by assessing properties such as abrasion and cut-through resistance, chemical and fluid resistance, dry and wet arc tracking, and flammability and smoke generation. At the same time, such cables must adhere to rigid weight limitations.
Aircraft wire constructions comprising a polyimide inner layer, and a polytetrafluoroethylene (PTFE) outer layer, are known. In such constructions, the polyimide inner layer is formed by spiral-wrapping an adhesive (e.g., PTFE, fluorinated ethylene-propylene (FEP), or perfluoroalkoxy (PFA))-coated polyimide tape, in an overlapping fashion, about a conductor. The spiral-wrapped polyimide tape is heat-sealed at the spiral-wrapped tape joints. The PTFE outer layer is formed by spiral-wrapping unsintered PTFE tape about the heat-sealed polyimide inner layer. The unsintered PTFE tape outer layer is also heat-sealed at the spiral-wrapped joints by sintering the wrapped tape.
The above-referenced aircraft wire constructions have a temperature rating of approximately 260xc2x0 C., and while demonstrating good mechanical durability, these wire constructions provide only low-to-moderate long-term humidity resistance and laser markability properties. In addition, the PTFE outer layer is easily scrapped off, thereby exposing the inner layer and rendering it susceptible to hydrolysis in humid environments.
As will be readily apparent to those skilled in the art, the aircraft wire constructions described above do not employ a radiation crosslinked outer layer, where exposing perfluorinated polymers such as PTFE, FEP, and PFA to radiation would serve to degrade these materials.
Aircraft wire constructions comprising one or more layers of extruded ethylene tetrafluoroethylene (ETFE) copolymer, are also known. In such constructions, the ETFE copolymer layer(s) is generally crosslinked by irradiation to achieve use-temperature ratings of greater than 150xc2x0 to 200xc2x0 C. The reduction in use-temperature ratings is partially offset by the fact that these wire constructions demonstrate mechanical durability, long-term humidity resistance, and laser markability properties which are superior to those noted above for polyimide/PTFE wire constructions.
A need therefore exists for an aircraft wire construction which qualifies for higher use-temperatures, while demonstrating improved mechanical durability, long-term humidity resistance, and laser markabilty properties.
It is therefore an object of the present invention to provide such an insulated wire construction.
It is a more particular object to provide a multi-layer insulation system for electrical conductors.
It is another more particular object of the present invention, to provide a lightweight insulated electrical conductor prepared using the above-referenced multi-layer insulation system, which qualifies for a temperature rating of up to approximately 230xc2x0 C., and which demonstrates improved mechanical durability, and hydrolysis resistance.
It is yet another more particular object to provide an insulated electrical conductor that further demonstrates flame resistance and laser markability.
It is a further object of the present invention to provide a process for preparing such an insulated conductor, and an insulated conductor prepared by such a process.
The present invention therefore provides a multi-layer insulation system for electrical conductors, which comprises:
(a) a polyimide or fluoropolymer inner layer,
wherein, when the inner layer is a polyimide inner layer, the layer is formed by wrapping a polyimide film, which has been coated with a sealable component, in an overlapping fashion, along a portion or length of an electrical conductor, wherein the polyimide film is substantially uniformly sealed to itself in overlapping regions along the length of the conductor, thereby forming an effective seal against moisture, wherein the sealable component comprises a perfluoropolymer, a crosslinked fluoropolymer, or a polyimide adhesive,
wherein, when the inner layer is a fluoropolymer inner layer, the layer is formed by either extruding a fluoropolymer material along a portion or length of the electrical conductor, or by wrapping a fluoropolymer film, in an overlapping fashion, along a portion or length of the conductor,
(b) optionally, a polyimide middle layer, wherein the polyimide middle layer is formed by wrapping an optionally coated polyimide film, in an overlapping fashion, along a portion or length of the inner layer formed on the electrical conductor, and
(c) an extruded, crosslinked fluoropolymer outer layer, wherein the fluoropolymer is selected from the group consisting of copolymers and terpolymers of ethylene-tetrafluoroethylene, and mixtures thereof,
wherein, when the inner layer is a fluoropolymer inner layer, the multi-layer insulation system includes a polyimide middle layer.
The present invention also provides an insulated electrical conductor that comprises an electrical conductor insulated with the multi-layer insulation system described above.
The present invention further provides a process for preparing an insulated electrical conductor, which comprises:
(a) forming a polyimide or fluoropolymer inner layer on an electrical conductor,
wherein, when the inner layer is a polyimide inner layer, the layer is formed by wrapping a polyimide film, which has been coated with a sealable component, in an overlapping fashion, along a portion or length of the electrical conductor, wherein the sealable component comprises a perfluoropolymer, a crosslinked fluoropolymer, or a polyimide adhesive,
wherein, when the inner layer is a fluoropolymer inner layer, the layer is formed by either: i) extruding a fluoropolymer material along a portion or length of the electrical conductor, or ii) wrapping a fluoropolymer film, in an overlapping fashion, along a portion or length of the electrical conductor,
(b) optionally, forming a polyimide middle layer on the polyimide or fluoropolymer inner layer by wrapping an optionally coated polyimide film, in an overlapping fashion, along a portion or length of the inner layer,
(c) when the inner layer is a polyimide inner layer or when a middle layer is formed using a coated polyimide film, heating the polyimide film or films to a temperature ranging from about 240xc2x0 to about 350xc2x0 C. to cause overlapping regions of the coated film or films to bond, thereby forming an effective seal against moisture along the length of the conductor,
(d) forming a fluoropolymer outer layer on either the inner or middle layer by extruding a fluoropolymer material along a portion or length of that layer; and
(e) crosslinking the fluoropolymer outer layer, wherein, when the inner layer or the sealable component comprises a perfluoropolymer (e.g., polytetrafluoroethylene, fluorinated ethylene propylene copolymers, perfluoroalkoxy resins), the fluoropolymer outer layer is crosslinked by exposing it to less than 60 megarads of radiation, with applied voltages ranging from about 50 to about 120 kilo volts,
wherein, when the inner layer is a fluoropolymer inner layer, the process for preparing an insulated electrical conductor includes forming a polyimide middle layer on the fluoropolymer inner layer.
The present invention also provides an insulated electrical conductor prepared by the process described above.
The foregoing and other features and advantages of the present invention will become more apparent from the following description and accompanying drawings.